This invention relates to a magnetoelectric device responsive to an applied magnetic field, which has particular but not exclusive application to a magnetic reading head for reading data from magnetic storage media.
Large magnetoresistance can be observed in certain structures that include regions of ferromagnetic material which are separated by regions of non-magnetic material. In these structures, the resistance drops dramatically as a magnetic field is applied, the change being much greater than for a single ferromagnetic film. The effect is believed to be due to relative alignment of the magnetisation directions of different layers of the structure. For example, considering a structure that comprises first and second ferromagnetic layers, when the magnetisation direction of the two ferromagnetic layers are aligned in anti-parallel, electrons with a particular spin can pass through one of the layers but are blocked by the other layer and cannot pass through it. However, in response to an applied magnetic field, the magnetisation direction of one of the layers can be made to flip into parallel with the other layer. Then, electrons with a spin orientation that can pass through the first ferromagnetic layer, will also freely pass through the second ferromagnetic layer, resulting in a relatively low resistance. The effect can be used to detect the presence of an applied magnetic field.
A giant magnetic tunnelling effect has been observed in cobalt containing ferromagnetic layers spaced apart by an insulating tunnelling barrier of aluminium oxide, as described by T. Miyazaka and N. Tezuka xe2x80x9cGiant Magnetic Tunnelling Effect in Fe/Al2O3/Fe junctionxe2x80x9d, J. Magn. Magn. Mater. 139, L231-L234. It has been proposed to use a magnetoelectric device of this general configuration in a magnetic reading head, as described in xe2x80x9cTMR as a promising device for third generation hard disk drive headsxe2x80x9d Nikkei Electronics 1997, 47 No. 686 pp 125-129. The magnetoelectric device used in the reading head comprises first and second overlying ferromagnetic layers formed of cobalt with a thickness of 3.3 nm, sandwiching a tunnelling insulating Al2O3 layer of thickness 1.3 nm, to act as a thin tunnelling barrier to channel electrons between the ferromagnetic layers. The various layers are deposited sequentially on an insulating substrate. The tunnel barrier of Al2O3 is formed by oxidising a thin aluminium layer deposited on one of the cobalt layers. The oxidisation process takes 10 hours or longer which slows the manufacturing process. Furthermore, the aluminium oxide film needs to have a high quality and uniformity in order to render the device sensitive to an applied magnetic field. Short circuits due to pin holes through the tunnelling barrier constitute a serious problem. Furthermore, the magnetoelectric device needs to be made sufficiently small to detect individual storage areas on magnetic media with the result that the resistance through the device is relatively high and sensitive to external noise.
The present invention seeks to provide an improved device which overcomes these problems.
According to the invention there is provided a magnetoelectric device responsive to an applied magnetic field, comprising first and second ferromagnetic regions with a channel region between them, the ferromagnetic regions being configured so that charge carriers with a particular spin polarisation which can pass through the first region, pass through the second region as a function of the relative orientations of magnetisation of the ferromagnetic regions produced by the applied magnetic field whereby the device exhibits a conductivity as a function of the strength of the applied field, wherein that the channel region is configured to provide a quasi-one-dimensional channel to cause charge carriers which pass through the first ferromagnetic region to maintain their spin polarisation as they pass towards the second ferromagnetic region.
The quasi-one-dimensional channel may comprise a nanotube, which may be formed of carbon. The channel region may comprise a bundle of such nanotubes.
The channel region in another aspect of the invention may comprise a layer of graphite or a diamond layer.
The invention also includes a magnetoelectric device responsive to an applied magnetic field, comprising first and second ferromagnetic regions with a channel region between them, characterised in that the channel region includes a carbon containing material.
The invention also includes a magnetoelectric device responsive to an applied magnetic field, comprising first and second ferromagnetic regions with a channel region between them characterised in that the channel region includes a nanotube.